The invention relates to a mobile radio system which comprises radio base stations to transfer control information and message information in the form of radio information to mobile terminals. The invention particularly relates to a method in which the transmission of message information to a mobile terminal is handed over from one radio base station to another radio base station, i.e. a so called handover.
It is desirable that a mobile radio system has a large traffic handling capacity and a high degree of coverage. The traffic handling capacity for a mobile radio system depends i.a. on the number of available radio channels and how effectively these channels can be utilized. It is known to provide several radio base stations having small coverage areas, also called cells, close to each other within a mobile radio system. Then, available radio channels can be utilized in a more efficient manner for handling traffic peaks within a restricted geographical area than if radio base stations having large coverage areas are arranged far from each other within the mobile radio system. The coverage area is the area within which a call is established. Thus, the provision of many radio base stations close to each other can increase the capacity in a mobile radio system. Two radio base stations with such overlapping coverage areas can not however normally use the same radio channel for communication with different mobile terminals which is true for e.g. mobile radio systems implemented in TDMA technology and FDMA technology.
When a mobile terminal has a communication link established to a first radio base station (below also called active radio base station and its associated cell is in analogy herewith called an active cell) and moves from the area that is covered by said first radio base station to another area which is covered by a second radio base station, a new communication link has to be established between the mobile terminal and the second radio base station. This is automatically controlled by the mobile radio system and the method is denoted handover or hand-off.
Comparatively much signaling is required in the mobile radio system in connection with handover. This means that the radio system is loaded every time handover is to be performed. Therefore, it is desirable to avoid having to perform unnecessary handovers.
Due to the presence of radio shadows caused e.g. by variations in the terrain, there can be islands in a cell belonging to one radio base station where the radio signal of the radio base station of another cell is stronger. If enough signal strength can be offered without handover being performed to such an island, it is disadvantage to perform such a handover. If handover is avoided to said islands, the signaling in a radio system is reduced and thereby a lower load on the mobile radio system is obtained.
In U.S. Pat. No. 4,718,081 an apparatus and a method are described for performing handovers. The object of the method is to avoid performing handovers to radio base stations which would not be optimal to perform handovers too. Signal strength measurement is carried out on handover candidate radio base stations and their neighboring radio base stations. Candidate radio base stations are the radio base stations to which it is possible for a mobile to perform a handover to if needed. Neighboring cells to a cell are the cells that surround this cell. A calculation of a weighted average value of measured signal strength for candidate radio base station and its neighboring radio base stations is carried out for all candidate radio base stations. The signal strength of the neighboring radio base stations are multiplied by a weight factor preferably equal to 0.5 in the calculation. The calculated weighted average values for the candidate radio base stations are compared. The radio base station having the highest weighted average value is selected to handle an ongoing call.
The present invention is based on the insight that, by taking into consideration how a second radio base station that possibly will take over a mobile call (handover) from a first radio base station, is related to the (other) radio base stations that are neighboring base stations to said first base station, it is possible to avoid unnecessary handovers when an island is present that is so located that it is not suitable to perform handover to the base station that has given rise to this island.
An object of the present invention is, thus, to avoid handovers in areas where radio communication signals are received with high signal strength (or where any other signal quality measure that is used to judge when handover is to take place, has a high value) from a radio base station that is far from said areas. Fewer handovers reduce the load on the radio system and also the risk for ongoing calls to be cleared down. This also leads to that the system will be less exposed to disturbances since the number of mobile terminals that are set up to the base station and that at the same time are far from the radio base station, is reduced.
This is attained by the present invention in that handover of a mobile terminal is performed from a first radio base station to a second radio base station when a first quality value, corresponding to a measured quality measure of a radio signal that has been transferred between the second radio base station and the mobile terminal, exceeds the same quality measure of the signal from the first radio base station added to a handover value that is dependent on a measured quality value of a radio signal that has been transferred between a further radio base station and the mobile terminal and a value on a neighboring cell relation (below called relation value).
An advantage of the present invention is that the number of handovers in the mobile radio system on an average is reduced which reduces the load on the mobile radio system.
The invention will now be described more in detail by means of preferred embodiments with reference to the appended drawings.